The present invention relates to a fuel assembly of a light water reactor, wherein the fuel assembly is charged with fuel rods in which a plutonium-uranium mixed fuel is packed.
It is possible to effectively utilize resource or improve fuel economy in a nuclear power plant by utilizing plutonium (Pu) obtained by reprocessing spent nuclear fuel in a light water reactor.
In a case where the plutonium is used as a fuel, since the nuclear characteristics of the plutonium fuel are different from those of the uranium fuel, the core and the core characteristics in case of using the plutonium fuel are also different from those in case of only using the uranium fuel. The main differences in the nuclear characteristics between the plutonium (Pu) and the uranium (U) reside in that (1) a thermal neutron absorption cross section of Pu-239 or Pu-241 of a fissile material of Pu is as large as twice that of U-235 of a fissile material of U and (2) a fissile material Pu-240 of Pu has a considerably large neutron absorption peak in a resonance region being an energy region in which the neutron is subjected to resonance absorption.
According to these facts, when Pu is charged in a reactor core, (1) neutron flux in a thermal region is decreased and the neutron flux in the resonance region or fast region is increased, that is, a neutron spectrum is hardened, and (2) on the basis of the increasing of the neutron absorption in the resonance region, the absolute value of the void coefficient (which is represented by a moderator temperature coefficient in a pressurized water reactor (PWR)) in a boiling water reactor (BWR), i.e., core reactivity change due to the change of the moderator density becomes large.
These facts affect the core characteristics in the following points manner.
(1) The core reactivity in a core cooling period is increased and the core shutdown margin is decreased. The reason why this is caused is represented by FIG. 14 as a mode.
(2) The core shutdown margin is decreased by the presence of a burnable poison such as gadolinium compound (gadolinia Gd.sub.2 O.sub.3) or boron (B) or the decreasing of a reactivity controlling ability of control rods.
(3) Particularly, in case of BWR in which the moderator density is largely changed in the core axial direction, the distortion of the power distribution in the core axial direction is made large in comparison with a core in which only the uranium fuel is used and the thermal margin is decreased. This reason will be explained with reference to FIG. 15.
Since Pu is a radioactive material and also has a chemical toxicity as a heavy metal, the formation process of the U-Pu mixture is carried out in a glove box. For this reason, the usage of less of the kind of fuel rods of different Pu enrichments is economical for the formation working or process and is also desirable for the management or handling.
For conveyance of the U-Pu mixture fuel, called hereinlater MOX fuel, assembly, it is necessary to convey the same by accommodating it in a container provided with a specific radiation shielding for preventing the radiation emitted from the Pu and in the nuclear power plant, the MOX fuel assembly is managed under a specific control. In this meaning, it is desired to utilize smaller number of MOX fuel assemblies also in view of economy.